Found this thread for the same reason. My cheapo Boardman was stolen in under an hour. I have seen two ads on Gumtree that could be it. Neither of the advertisers had a profile, and they didn't respond to my messages or texts at all. The police have been very nice but useless.
Two months later I'm still livid. This is not just about super rich people with massively expensive bikes, mine was the bottom of the range, £370 Boardman that I bought in the sale and those a*holes took my only means of transport. It just happened to have a label that sells. I wasn't insured. Noone agrees to a £200 excess for a cheap bike. You just buy another cheap bike (probably stolen).
I also just found a thread here from *2011* suggesting Gumtree put in a verification process. 8 years later and it is still ridiculously easy to shift stolen bikes there.
How hard could it be to force Gumtree to verify each new user? Or to introduce an official, national frame number register? Or pet microchips for bikes?
Are there better ideas other than making your bike ugly?